Most lubricant circulation systems contain some form of filtration to remove debris from the lubricant to extend component life. As the filter ages and becomes more loaded with particulate, it becomes more restrictive thereby requiring greater line pressure to cause the lubricant to flow through the filter at the same flow rate. The differential pressure (ΔP) across the filter is therefore a reasonable indicator of the filter's loading level and remaining useful life. Filter differential pressure monitoring is widely utilized on aircraft and large trucks to assess when a filter is fully loaded and requires replacement.
Monitoring the level of debris in a lubricant circulation system can provide an indication of abnormal component wear requiring maintenance action. In this regard, component wear in a propulsion system (e.g., wear associated with bearings, gears, shafts and the like) is typically evidenced by particulate metal that has been removed from the component through wear and deposited in a circulating lubricant (e.g., oil). Thus, by analyzing the type and amount of metal particles in a propulsion system's circulating lubricant, an operator can assess the relative health of the propulsion system components to facilitate maintenance decisions.
Several prior methods for detecting wear processes and failure progression of propulsion systems that include bearings and gears require intrusive instrumentation, large high frequency data sets, and sophisticated analysis methods. For example, vibration monitoring is a prominent method for continuously assessing the relative health of propulsion system components. However, vibration monitoring requires vibration sensors to be distributed throughout the system with the need for access of the various vibration sensor components and the ability to isolate and interpret the vibration responses. These requirements hinder or preclude the use of vibration techniques in many applications. Also significant damage to the propulsion system may occur before such damage is detected through vibration monitoring thereby giving operators little time to react to impending failure.
Recently U.S. Pat. No. 7,299,683 to Nikkels et al (the entire content of which is expressly incorporated hereinto by reference) has disclosed a metal particle sensor to detect presence of metal particles in the lubricant flow. However, since the metal particle sensor is sensitive to lubricant flow rate, it must be corrected for the effects of the local flow rate in order to provide an accurate particulate measurement. In order to correct for such flow rate sensitivity, the system of Nikkels et al '683 suggest using a differential pressure measurement across an orifice to measure flow rate in proximity to the metal particle sensor. The combined use of a metal particle sensor and a differential pressure across an orifice thereby serves to correlate the output of the metal particle sensor to the flow rate as determined by the differential pressure across the orifice.
Other techniques for detecting wear debris in lubricant systems include magnetic chip collectors. However, such magnetic chip collectors require operators to periodically remove the collected chips to allow for physical inspection and determination of the type and amount of wear debris that has been collected. While magnetic chip collectors overcome the complexity of implementing a vibration sensor system, they do not provide the operator with real time data that can be used to warn of impending component failure. Also chip detectors only collect particles that come into proximity of the magnetic collection element which are known to have poor collection efficiencies.
Systems which employ a single differential pressure measurement have been used to monitoring the amount of debris collected by a filter element (colloquially known as “filter loading”) so as to maintain the filter element in good condition. Such systems however are incapable of assessing the type and/or size of the debris removed from the fluid by the filter element, especially in the small quantities associated with impending component failure. As such, such single differential pressure measurement systems cannot determine if the collected debris is abnormal and/or potentially associated with failure progression of a component. In addition, such conventional single differential pressure measurement systems are also not sufficiently sensitive to detect the early stages of wear damage to components associated with a propulsion system, such as bearings, gears or other engine wear parts.
Moreover, while the use of single differential pressure measurements is known for detecting filter loading, such a technique is dependent on both fluid temperature and flow rate. The required measurement system resolution for use in detecting minute amounts of wear debris could therefore only be achieved through extremely accurate flow rate and temperature measurements at the filter element(s).
It would especially be desirable if self-compensating detection systems and/or methods could be provided that were capable of detecting debris in a circulating fluid that is independent of flow rate and/or temperature (viscosity, density) of the fluid so that the wear processes and failure progression of components could be determined in real time. It is towards providing such systems and methods that the present invention is directed.